Most human tumor cell strains are capable of supporting the growth of adenoviruses which have been treated with carcinogenic compounds such as N-methyl-N'-nitro-N-nitrosoguanidine and methylnitrosourea which alkylate DNA. About one-fifth of the cell strains tested are significantly deficient in supporting growth of the damaged viruses. We interpret this as a deficiency in DNA repair. The ability of cell strains to recognize and incise alkylated DNA are studied in extracts of cells acting on bacterial plasmid DNA alkylated in vitro. A technique to measure this incision using DNA affinity chromatography has been developed. The kinetics of incision by extracts of cells are being studied. In addition, we are studying repair replication in cell strains following treatment with DNA alkylating agents, using the technique of photolysis of bromodeoxyuridine (BUDR) incorporated during repair.